No, I am not aware of any fields that could be used to calculate total update size for a given selection of updates.
Yes, Planning Mode allows the execution of an update management task without actually installing any updates. This allows you to perform a 'what-if' analysis that shows what updates would be installed based on the rules and options selected. The task will show up in Task History where one can review the Details tab to see a list of the Operations that would be performed and the results expected. Some examples:
" The update 'UpdateName' for IE 8 for windows 2008 (KBxxxxxx) with classification 'Security Updates' and ID 'updateID' with revision ID '102' was requested for install is already installed. "
or ..."was requested for install would be attempted to be installed."
or, in some cases, would just show "No applicable updates were matched."